Polypropylene is often a material of choice for medical articles due to its various properties such as non-toxicity, chemical resistance and inertness to drugs and liquid media used with drugs, as well as its low cost and ease of processing by means of extrusion, molding, and the like.
However, a disadvantage of crystalline polypropylene is its inherent inability to be heat sealed against another material. Medical articles or packaging for medical articles often require heat sealing in the manufacturing process to assemble the components of the article or the packaging process to protect the active ingredient or medical article in the packaging from undesired exposure.
Medical articles requiring additional protection beyond secure heat sealing in manufacturing or packaging processes can be sterilized at the time of production and thereafter maintained in a sterile condition during storage. Not all medical articles require sterilization prior to usage. But structural components resistant to radiation are more versatile for uses in medical articles and packaging than components unable to maintain structural integrity after irradiation. Thus, the most desirable material for a medical article or the packaging therefor is one which possesses resistance to the structurally demanding forms of sterilization even if current usages of the medical articles do not require such sterilization.
A preferred method of sterilization uses gamma radiation, such as radioactive cobalt 60, since it can be performed on packages sealed by heat or other methods insuring total and reliable sterility of the contents.
Unfortunately, gamma-irradiation of crystalline polypropylene causes degradation of its structural integrity (e.g., causing embrittlement, discoloration, thermal sensitivity).
To avoid such degradation, the prior art has employed a variety of stabilizers and other additives. European Patent Publication 0248545 (assigned to the same assignee as for this application), the prior art therein described the efforts undertaken to stabilize crystalline polypropylene from degradation after irradiation.
Conversely, mesomorphous, non-crystalline polypropylene, as described in European Patent Publication 0248545, provides resistance to sterilization irradiation without the necessity of any additive or stabilizer. Control over the method of preparing such mesomorphous polypropylene causes such polypropylene to substantially maintain its structural integrity after sterilization irradiation.
Unfortunately, films for packaging and the like made from polypropylene, even non-crystalline mesomorphous polypropylene, are susceptible to tearing and puncturing that disrupt maintenance of structural integrity as a manufactured product or packaging component after assembly. Usefulness of a sterilized medical article is compromised by a puncture in a polypropylene package. Also, as described above, crystalline polypropylene is not a material which can be heat seal ed against another material in order to provide either a multi-component medical article or an effective radiation sterilized package.
On the other hand, polybutylene, poly(1-butene) or poly(2-butene) or both, offers many advantages to the medical packaging art that polypropylene, mesomorphous or otherwise, lacks. Polybutylene has high tear strength, high impact strength, puncture resistance. Polybutylene is also often used as a film requiring heat sealability. Reference to the versatility of polybutylene may be found in Shell Technical Bulletin sc: 391-79 (1979). However, polybutylene is highly crystalline (as much as 98% crystalline). After irradiation, the melt index of polybutylene has increased, indicating chain scission. See Bradley, Journal of Industrial Irradiation Technology (2) 93-138 (1984). Hence, polybutylene packaging usually degrades over the effective storage life often needed for a medical product.
Neither crystalline polyproylene nor crystalline polybutylene substantially maintain structural integrity after sterilization irradiation. Mesomorphous polypropylene lacks desirable strength packaging properties. No single polymer combines radiation resistance and several good packaging properties, e.g. tear strength, puncture resistance, and heat sealability.
Thus, what is needed is a heat sealable medical packaging material having good strength which substantially maintains its structural integrity for a useful period, even after exposure to the irradiation dosages necessary to sterilize such material.